Catalysts for polymerization or copolymerization of ethylene, which include magnesium, are known to have very high catalytic activity and provide polymers of high bulk density, and are suitable for liquid phase or gas phase polymerization. Liquid phase polymerization of ethylene is a polymerization process performed in a medium such as bulk ethylene, isopentane or hexane. The important features of the catalysts for the processability in this process are catalytic activity, bulk density of the resultant polymers, and the amount of low molecular weight material melted into the medium. The molecular weight distribution of the polymers depends on the characteristic of catalyst itself and the polymerization process. The polymers which are produced by using Ziegler-Natta type catalysts in a single reactor, as a slurry or in the gas phase, generally have narrow molecular weight distribution. Because of this molecular structure, processability of polymers is lowered and tensile strength and rigidity in the molten state is limited, thereby resulting in the distortion, contraction of shape and Parrison deflection during processing. Moreover, these polymers are difficult to apply to the production of large scale pipes or large blow molding products which require high mechanical resistance in a molten state. When the molecular weight of the producing polymer is increased, it has an advantage of increasing the tensile strength, however is more difficult to process. This causes problems such as cracking during processing. In order to overcome these problems, it is desirable to increase the molecular weight while having a broad molecular weight distribution since excellent processability can be obtained while maintaining high tensile strength.
Many titanium-based catalysts containing magnesium for olefin polymerization, and the manufacturing methods thereof have been reported. Specifically processes making use of magnesium solutions to obtain catalysts which can generate olefin polymers of high bulk density are known. Magnesium solutions may be obtained by reacting magnesium compounds with such electron donors as alcohols, amines, cyclic ethers, or organic carboxylic acids in the presence of a hydrocarbon solvent. The formation of magnesium solutions by the reaction of magnesium compounds with an alcohol is disclosed in U.S. Pat. Nos. 3,642,746, 4,336,360, 4,330,649 and 5,106,807. Further, methods for the production of catalysts containing magnesium by reacting a liquid-phase magnesium solution with a halogenated compound, such as titanium tetrachloride, are well known. Moreover, there have been attempts to control polymerization activity or molecular weight distribution of polymers by adding ester compounds. Such catalysts provide high bulk density to the resultant polymers, but their catalytic activities or the molecular weight distribution of the resulting polymers could be improved. Moreover, tetrahydrofuran, a cyclic ether, has been used as a solvent for a magnesium compound in U.S. Pat. Nos. 4,477,639 and 4,518,706.
Meanwhile, U.S. Pat. Nos. 4,847,227, 4,816,433, 4,829,037, 4,970,186, and 5,130,284 have reported the use of such electron donors as dialkylphthalate, phthaloyl chloride, etc. in reaction with a titanium chloride compound for the production of olefin polymerization catalysts having improved polymerization activity, and which are capable of enhancing the bulk density of resultant polymers.
U.S. Pat. No. 5,459,116 has reported a method of production of a titanium solid catalyst by contact-reacting a magnesium solution containing an ester having at least one hydroxyl group as an electron donor with a titanium compound. By this method, it is possible to obtain a catalyst having high polymerization activity, which accords high bulk density to resultant polymers, but there is room for yet more improvements.
U.S. Pat. No. 3,899,477 has disclosed a catalyst which is prepared from titanium halide, vanadium halide and organoaluminum compounds. This catalyst can be used to produce polymers having broad molecular weight distributions by treating the catalyst with alkylaluminum sesquiethoxide and trialkylaluminum before polymerization. This prior art, however, has disadvantages in that the catalyst production process is complicated and it is difficult to control the polymerization process conditions due to the differences between titanium and vanadium reactivity to hydrogen, monomers and co-monomers.
As described above, it is very difficult to produce polymers with broad molecular weight distribution by using common Ziegler-Natta type catalysts while maintaining high catalytic activity and high bulk density of resultant polymers. Moreover, conventional preparation techniques for catalysts tend to be complicated and such catalysts tend to make control of polymerization processes difficult. Consequently, a method for controlling the molecular weight distribution of polymer is needed in order to expand the use of ethylene polymer and to improve their processability and physical properties.